1. Field of the Invention
This invention relates to a radiation image read-out and gradation processing method for use in a radiation image recording and reproducing system for medical diagnosis or the like, and an apparatus for carrying out the method.
2. Description of the Prior Art
A novel radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. The system comprises the steps of: (i) scanning a stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, (ii) detecting the emitted light and converting it into an electric image signal by a photoelectric read-out means, (iii) subjecting the electric image signal to a predetermined gradation processing suitable for image viewing, particularly for diagnostic purposes, and (iv) reproducing a visible image on a photographic material or the like on the basis of the gradation-processed image signal.
In the aforesaid radiation image recording and reproducing system, the electric image signal obtained by the photoelectric read-out means in proportion to the amount of the light emitted by the stimulable phosphor sheet is subjected to a predetermined gradation processing for obtaining a visible image or a signal for the visible image suitable for the image recording portion of the object and/or image recording conditions, i.e. suitable for viewing, particularly for diagnostic purposes. The visible image is reproduced on the basis of the gradation-processed electric image signal. (The operation for detecting the electric image signal from the light emitted by the stimulable phosphor sheet by the photoelectric read-out means for obtaining the visible image is hereinafter referred to as the final read-out.) The range of the level of radiation energy stored in the stimulable phosphor sheet is different in accordance with the image recording portion of the object, e.g. the lungs or the heart, and/or image recording conditions used, such as plain image or contrasted image recording. Therefore, when the electric image signal is directly detected from the light emitted by the stimulable phosphor sheet in the final read-out, the electric image signal of a level range which differs in accordance with the image recording portion and/or image recording conditions is fed to the next gradation processing step. This is not advantageous.
Accordingly, in order to feed an electric image signal of approximately the same level range regardless of the image recording portion and/or image recording conditions to the gradation processing step, the aforesaid radiation image recording and reproducing system is constructed so that the final read-out is conducted on the basis of a fixed read-out condition GPO which is predetermined in accordance with the image recording portion and/or image recording conditions.
However, even when the image recording portion and/or image recording conditions are the same, for example, even in the case of plain image recording of the chest, the level range of radiation energy stored in a stimulable phosphor sheet differs in accordance with the thickness of the chest. In this case, if the gradation processing is conducted by use of gradation processing conditions (nonlinear transformation function) predetermined to suit the image recording portion and/or image recording conditions, a problem that arises is that it becomes difficult to grasp all of the effective image information stored in the stimulable phosphor sheet.
To solve the aforesaid problem, an improved method has been proposed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the method, a read-out operation for approximately detecting the image information such as the range of the level of radiation energy stored in the stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out, and thereafter the final read-out is carried out. A final read-out condition (read-out scale factor) GP is decided on the basis of the image information obtained by the preliminary read-out, and an electric image signal is obtained from the light emitted by the stimulable phosphor sheet in the final read-out by use of the read-out condition GP.
As described above, the level of the stimulating rays used in the preliminary read-out should be lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out, the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like positioned on the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
In the method conducting the preliminary read-out prior to the final read-out, it is possible to obtain an electric image signal processed in advance by use of the read-out condition set to suit the effective image information stored in the stimulable phosphor sheet at the final read-out step, and to feed the electric image signal to the next gradation processing step. Therefore, the gradation processing can be conducted easily without losing the effective image information. As a result, it is possible to easily obtain a visible image having an image quality suitable for viewing, particularly for diagnostic purposes. The function of deciding the read-out condition on the basis of the image information obtained by the preliminary read-out is hereinafter called the automatic sensitivity adjusting function.
However, the read-out and gradation processing system provided with the automatic sensitivity adjusting function has a drawback as described below. For example, in magnification image recording of the lungs, the density range of the lungs in a reproduced visible image where only the image of the lungs is recorded is different from where the image of the lungs is recorded together with that of a bone or the heart, and the image quality of the reproduced visible image becomes low.
FIGS. 1A and 1B are explanatory views showing the density condition of a visible image reproduced by the conventional radiation image recording and reproducing system provided with the automatic sensitivity adjusting function. FIG. 1A is for the case where only the image of the lungs is recorded, and FIG. 1B is for the case where the image of the lungs is recorded together with that of the heart. When only the image of the lungs is recorded in magnification image recording of the lungs conducted by use of the read-out and gradation processing system provided with the automatic sensitivity adjusting function, the amount of X-ray energy stored in the stimulable phosphor sheet is as shown by a histogram curve P in FIG. 1A and is detected in the final read-out by use of the read-out condition (read-out scale factor) GP decided by the preliminary read-out. The electric image signal obtained by the final read-out step is directly fed to the gradation processing step in which the electric image signal is gradation-processed by use of the predetermined gradation processing condition (nonlinear transformation function) GH for magnification image recording of the lungs. The signal obtained by the gradation processing is sent to an image reproducing step and is used to reproduce a visible image on a photographic material or a display device in accordance with gradation characteristics GL at the image reproducing step. In this manner, all of the effective image information is expressed within a predetermined density range D1 to D2 in the visible image.
In general, the aforesaid image reproducing condition GL is of a fixed value inherent to the image reproducing step, i.e. it is fixed by a reproducing apparatus or the like. Also, though the gradation processing condition GH is different in accordance with the image recording portion and/or image recording conditions, it is a fixed value when the image recording portion and/or image recording conditions are the same. For example, in the case of magnification image recording of the lungs, the gradation processing condition GH is a fixed value which is adjusted to suit the magnification image recording of the lungs. Since GH and GL are fixed, in order to obtain a visible image wherein all of the aforesaid effective image information is expressed within the density range D1 to D2, it is necessary to adjust the whole range of the electric image signal generated by the final read-out step to a predetermined range corresponding to the density range D1 to D2. For this purpose, the read-out condition GP is changed by the automatic sensitivity adjusting function to an appropriate value in accordance with the condition of the effective image information.
However, when the image of the lungs is recorded together with that of the heart in the magnification image recording of the lungs, the amount of X-ray energy stored in the stimulable phosphor sheet becomes as indicated by a histogram curve Q in FIG. 1B. In this case, the range of the X-ray amount is extended from a range R of the lungs only to a range S of the heart, and the automatic sensitivity adjusting function automatically adjusts the read-out condition GP so that the extended range R+S corresponds to the predetermined density range D1 to D2. Accordingly, as shown in FIG. 1B, the density range of the lungs in this case becomes D3 to D2, and becomes narrower than the density range D1 to D2 in the case where only the image of the lungs is recorded. Therefore, the image quality, particularly with regard to diagnostic efficiency and accuracy, of the lungs in the visible image reproduced becomes low.